Human activities produce a large amount of nitrogen oxides (NOx) mainly including NO and NO2, of which 90% are produced by fossil fuel combustion, followed by nitric acid manufacturing process, nitrification of chemical and pharmaceutical industries, metal surface and semiconductor treatment processes. NOx has a toxic effect on humans, and the emission of a large amount of nitrogen oxides is one of the main causes of atmospheric photochemical smokes and acid rains. China statistical yearbook on environment (2010) showed that the contribution of nitrogen oxides to China's urban acid rain increased in some regions. Therefore much more stringent laws and regulations on the emissions of nitrogen oxides have been enacted, especially for the emissions from thermal power plant and other fossil combustion processes.
In general, the concentration of nitrogen oxides in exhaust gases from fossil fuel combustion, such as at thermal power plants, is about several hundred to several thousand parts per million (ppm), of which more than 95% is nitric oxide (NO). At present, selective catalytic reduction (SCR) is one of the major methods to remove NOx from flue gas. But the catalyst works at strict operating conditions, and ammonia is needed as the reducing agent. When the flue gases contain sulfides and dusts, the catalyst may be degraded for a period of operation. Operation of coal-fueled thermal power plants results in particularly high SCR system operating costs. The main method for emission source purification at low temperatures is a wet method using a variety of liquids to absorb NOx from gas stream. There are two kinds of oxidation absorption and reduction absorption for NOx removal. An oxidation method is the use of hydrogen peroxide, sodium hypochlorite and potassium permanganate as an oxidant. A reduction method is the use of sodium sulfite, sodium sulfide and urea as a reducing agent. However, when the flue gas contains much more nitric oxide, due to the low solubility of nitric oxide in solution, the removal efficiency is low, and the operating costs are also high due to the consumption of expensive reactants. Therefore a new nitrogen oxide purification technology is desired.